The present invention relates to control circuits for providing direct current (DC) control voltages and, more particularly, to a control circuit for controlling the gain of an attenuator by providing a variable DC control voltage.
Attenuators are used in a myriad of applications for varying the amplitude of a signal coupled therethrough as the gain thereof is varied. For example, contemporary speakerphones providing half-duplex voice communication typically comprise an attenuator in both the transmit and receive signal paths to enable the half-duplex operation as is understood. The MC34018, manufactured by Motorola Inc., is such a speakerphone wherein the transmit and receive attenuators are operated in a complementary manner and are controlled by a single control circuit providing a DC control voltage which varies the gains of the two attenuators respectively.
A problem with some known attenuator controller circuits is that the DC control voltage is generated independently of the gain of the attenuator. Thus, there is no feedback therebetween to precisely adjust the control voltage to provide good control of the gain of the attenuator. In fact, most if not all known speakerphones use such an open loop control scheme for controlling the gains of the transmit and receive attenuators which is not desirable as the control voltage is not generated accurately.
Hence, a need exists for a control circuit for accurately generating a DC control voltage that is the function of the attenuator gain which is varied by the control voltage.